The present invention relates to an adaptive threshold circuit in which a threshold level automatically changes in accordance with varying reference conditions, and more particularly to an adaptive threshold circuit suitable for use in an optical character recognition (OCR) machine. As described herein, the present invention constitutes a modification utilized in the OCR machine described in each of the following U.S. Patents and Applications which are incorporated herein by reference: U.S. Pat. No. 3,812,459; U.S. Pat. No. 3,872,433, and application Ser. No. 382,365, filed July 25, 1973, by Harrison B. Lidkea and Thomas G. Holmes, and entitled "Method and Apparatus for Recognizing Handwritten Characters In An Optical Character Recognition Machine".
The OCR machine described in the aforementioned Patent Applications employs a low power laser beam which scans across lines of a document to sequentially illuminate individual text character slices. Each illuminated slice is reflected by the scanning mirror back to a linearly extending array of photodiode elements. The scanning mirror is substantially centered relative to the document page so that the optical path subtends different angles relative to the document page and changes in length as a function of scan angle. More particularly, the mirror scans an angle of 15.degree. on each side of the vertical center line of the document page. The reflected light from the document varies as a function of the fourth power of the cosine of the scan angle. Therefore the light gathered by the mirror at the end of a scan is reduced by approximately 13% in comparison with the light gathered by the mirror at the middle of the scan.
The photodiodes in the array drive individual amplifiers which provide output voltages for comparison with a threshold level to determine whether a character or non-character portion of the document is being viewed by each photodiode. The output voltage is at a relatively high reference level when a photodiode has a non-character or white portion of the document projected thereon. The voltage falls in response to projected character portions, the voltage level being lowest for the blackest or most non-reflective characters. Since the light gathered by the mirror changes during the scan, the photodiode amplifier output signal level is dependent upon scan position. In the system described in the aforementioned patent applications, the threshold level is maintained constant throughout a complete scan. Since the photodiode signal representing the projected image from the document varies as a function scan angle but the threshold level remains constant throughout the scan, the sensitivity of the detection circuitry is less at the ends of the scan. This characteristic may become particularly disadvantageous when sensing handwritten characters, especially those written with lead pencil, since such characters are more highly reflective than most standard machine print characters; that is, handwritten characters produce signal levels which are closer to the reference levels produced by the non-character portion of the document page. Under such circumstances the threshold level must be closer to the reference level. As mentioned previously, this reference level changes as a function of scan angle and consequently comes fairly close to the constant threshold level. It is therefore possible for signal noise to interfere with reliable threshold detection at the ends of the scan intervals.
The foregoing problem, of course, is not limited to optical character recognition machines but is present in any threshold detection arrangement wherein a reference condition is subject to change.
It is therefore an object of the present invention to automatically compensate a threshold detection circuit for changes occurring in a reference condition.
It is another object of the present invention to provide an adaptive threshold detection arrangement in which a threshold level is automatically varied in accordance with variations occurring in a reference level.
It is a more specific object of the present invention to provide a threshold detection arrangement in an optical character recognition machine wherein the threshold level is automatically compensated for changes occurring in the light-gathering capability of the optical system as a function of scan angle.
It is still another object of the present invention to provide an adaptive threshold detection system for use in an optical character recognition machine whereby the threshold level for character detection is automatically varied in accordance with the light-gathering capability of the machine at different scan positions.